Polypyridobisimidazole polymer is a rigid rod polymer. Fiber made from this polymer, one polymer composition of which is referred to as PIPD and known as the polymer used to make M5® fiber, is known to be useful in both cut and flame resistant protective apparel. See, for example, PCT Application WO199902169 and WO2005002376. Fibers made from rigid-rod polymers having strong hydrogen bonds between polymer chains, e.g., polypyridobisimidazoles, have been described in U.S. Pat. No. 5,674,969 to Sikkema et al. An example of a polypyridobisimidazole includes poly(1,4-(2,5-dihydroxy)phenylene-2,6-pyrido[2,3-d:5,6-d′]bisimidazole), which can be prepared by the condensation polymerization of tetraaminopyridine and 2,5-dihydroxyterephthalic acid in polyphosphoric acid. Sikkema describes that in making one- or two-dimensional objects, such as fibers, films, tapes, and the like, it is desired that polypyridobisimidazoles have a high molecular weight corresponding to a relative viscosity (“Vrel” or “ηrel”) of at least about 3.5, preferably at least about 5, and more particularly equal to or higher than about 10, when measured at a polymer concentration of 0.25 g/dl in methane sulfonic acid at 25° C. Sikkema also discloses that good fiber spinning results are obtained with poly[pyridobisimidazole-2,6-diyl(2,5-dihydroxy-p-phenylene)] having relative viscosities greater than about 12, and that relative viscosities of over 50 (corresponding to inherent viscosities greater than about 15.6 dl/g) can be achieved.
Published U.S. Patent Applications US20030228812 and US20040216215 and U.S. Pat. No. 6,624,096 disclose protective fabrics & garments made from blends of Kevlar® and polybenzimidazole (PBI). U.S. Patent Application No. 2003/0228812 discloses the use of polybenzimidazole (PBI) and PBO fibers in protective apparel. PCT Patent Application No. WO2004023909 discloses the use of polybenzimidazole and poly(paraphephenylene benzobisaxazole) fibers and filaments in fabrics.
The aforementioned polybenzimidazoles are polybibenzimidazole compositions, which are not rigid rod polymers. Therefore the fiber made from that polymer has low fiber strength.
Thermal and flame retardant protective apparel has been used by firefighters, emergency response personnel, members of the military and racing personnel to save lives and reduce injury due to fires and other thermal events. Polypyridobisimidazole fiber has excellent fire resistant properties, superior in many respects to most other fibers, and because of the rigid rod nature of the polymer, fibers made from polypyridobisimidazoles have exceptionally high strength. There is, however, a desire to incorporate the superior fire resistance of polypyridobisimidazole fibers into outer shell fabrics for protective garments to take advantage of their superior flame resistant properties and improve their strength and durability in extreme conditions.